1. Field of the Invention
This invention generally relates to a photo-coupler apparatus, and more particularly, to a photo-coupler apparatus having at least one MOSFET as an output contact.
2. Description of the Prior Arts
FIG. 1 shows the circuit diagram of a prior art photo-coupler apparatus (first prior art). This prior art has been published in Japanese unexamined patent publication TOKUKAISHO 57-107633 which corresponds to U.S. Pat. No. 4,390,790, and EPS 0048146.
The photo-coupler apparatus of the first prior art has a light emitting diode 1 at the primary side. The secondary side of this device is comprised of the following: a first photoelectromotive diode array 2a photo-coupled with light emitting diode 1; a second photoelectromotive diode array 2b photo-coupled with light emitting diode 1; a resistor 3 parallel-connected with second diode array 2b; a normally-ON drive FET 5; and an output MOSFET 4. In this figure, numbers 8 and 8' show the input terminals and numbers 9 and 9' show the output terminals of this photo-coupler apparatus.
As shown in FIG. 1, the photo-coupler apparatus has two photoelectromotive diode arrays 2a and 2b. Once the second photoelectromotive diode array 2b has received a light from light emitting diode 1, it creates an electromotive force. Because of this electromotive force, drive FET 5, which is normally 0N, turns off. Then, the gate-source capacitor of output MOSFET 4 is rapidly charged by the photo-current from first photoelectromotive diode array 2a.
Once the incident light from light emitting diode 1 is interrupted, the accumulated charges between the gate and source of drive FET 5 are discharged through resistor 3, thus permitting drive FET 5 to turn on. At this time, the charges accumulated between the gate and source of output MOSFET 4 are rapidly discharged via the source and drain of drive FET 5 which is now in an ON state, allowing MOSFET 4 to turn off quickly.
As described above, the photo-coupler apparatus of the first prior art should include second photoelectromotive diode array 2b so as to control drive FET 5. Due to the existence of array 2b, the chip area of this apparatus increases, thus also increasing its manufacturing cost.
In FIG. 2A, the circuit structure of a photo-coupler apparatus is shown according to the second prior art of this invention. This apparatus has been published in Japanese unexamined patent publication TOKUKAISHO 63-99616 which corresponds to U.S. Pat. No. 4,873,202.
The second prior art photo-coupler apparatus has a light emitting diode 1 at the primary side. The secondary side of this apparatus is comprised of the following: a photoelectromotive diode array 2 which is photo-coupled with light emitting diode 1; an impedance element 6 connected in series with array 2; normally-ON drive FET 5; and output MOSFET 4 connected to output terminals 9 and 9'.
In this photo-coupler apparatus, once photoelectromotive diode array 2 receives a light from light emitting diode 1, a current flows through impedance element 6. Due to this current, a voltage difference, which can activate FET 5, arises between the source and gate of FET 5. Therefore, this apparatus does not need second photoelectromotive diode array 2b shown in FIG. 1 for the activation of drive FET 5.
In this case, however, impedance element 6 limits the charging current for the capacitor of MOSFET 4 when the resistance value of element 6 is large. This fact makes the charging period for MOSFET 4 longer. Thus, the time T-on, which is the period from the signal input to the turn-on of output MOSFET 4, becomes longer.
Impedance element 6 also works as a discharge resistor for discharging the accumulated charges at the source and gate of FET S. Therefore, if the resistance value of impedance element 6 is large, the discharging period of FET 5 becomes longer. As a result, time T-off, which is the period from the cut off of an input signal to the turn-off of output MOSFET 4, becomes longer.
On the contrary, if the resistance value of impedance element 6 is small, the current flowing through impedance element 6 must be increased to obtain a sufficient voltage for the activation of drive FET S. Therefore, the magnitude of minimum input current I-ft, which is required to turn on MOSFET 4, becomes larger, thus deteriorating the dynamic sensitivity of this device. As is evident from the above mentioned explanation, there is a trade-off between the switching times (T-on and T-off) and the minimum input current I-ft of MOSFET 4. This trade-off prevents the complete improvement of the characteristics of this photo-coupler apparatus.
FIG. 2B shows the characteristic curves of the second prior art photo-coupler apparatus. The detail of this figure will be explained later in conjunction with one embodiment of this invention.
In FIG. 3A, the circuit structure of a photo-coupler apparatus is shown according to the third prior art of this invention. This apparatus has been published in Japanese unexamined parent publication TOKUKAISHO 63-153916 which corresponds to U.S. Pat. No. 4,801,822.
This apparatus has a light emitting diode 1 at the primary side. The secondary side of this apparatus is comprised of the following: a photoelectromotive diode array 2 which is photo-coupled with light emitting diode 1; impedance element 6' including a resistor 6a and a zener diode 6b connected in parallel with each other; normally-0N drive FET 5; and output MOSFET 4 connected to output terminals 9 and 9'.
In this photo-coupler apparatus of the third prior art, impedance element 6' is comprised of the parallel circuit of resistor 6a and zener diode 6b as mentioned above. Because zener diode 6b works as a bypass for resistor 6a, most of the photo-current from array 2 flows through zener diode 6b, not through resistor 6a, when the current amount is large. As a result, resistor 6b does not limit the amount of charging current for MOSFET 4. This fact permits resistor 6b to have a greater value of resistance without making T-on longer. Time T-on and minimum input current I-ft can, therefore, be improved simultaneously in the third prior art.
In said case, however, there are still some problems. That is, an extra component, zener diode 6b, is necessary to construct the photo-coupler apparatus. And, if resistor 6b has a large value of resistance so as to reduce the amount of input current I-ft (that is, to improve the dynamic sensitivity), time T-off becomes longer.
FIG. 3B shows the characteristic curves of the third prior art photo-coupler apparatus. The detail of this figure will be explained later in conjunction with another embodiment of the present invention.
To summarize the above mentioned results, the photo-coupler apparatus of the prior arts have the following disadvantages:
(1) the photo-coupler apparatus having two photoelectromotive diode arrays (in the first prior art) requires an additional chip area for the installation of the second photoelectromotive diode array in order to activate a drive FET, thus increasing the chip area as well as the manufacturing cost; PA1 (2) although it does not require the second photoelectromotive diode array, the second prior art photo-coupler apparatus, in which an impedance element is series-connected with a photoelectromotive diode array, presents a trade-off relation between the switching times (T-on and T-off) and the minimum current I-ft (that is, sensitivity), thus preventing the improvement of the entire characteristics of this apparatus; PA1 (3) the third prior art photo-coupler apparatus, in which an impedance element is comprised of a resistor and a zener diode parallel-connected each other, can improve both T-on and I-ft simultaneously, but it requires an extra component, zener diode 6b, and time T-off becomes longer if a large value of resistor is used to improve I-ft.